1. Field of the Invention
The present invention relates to an optical diffusion sheet used in order to suppress decrease of the screen visibility of a visual display, such as in liquid crystal displays (LCD), organic EL displays and PDP, and to an optical element having the optical diffusion sheet concerned prepared thereto. Moreover, the present invention also relates to a visual display with the optical diffusion sheet or the optical element concerned provided thereto.
2. Description of the Prior Art
Conventionally, in image viewing displays, such as LCD, an indoor lighting such as a fluorescent light, or incidence from sunlight from windows, or a reflection of operator""s shadow etc. given to the viewing display surface sometimes aggravates visibility of pictures. Therefore, an optical diffusion layer is provided, on display surface, in which fine concavo-convex structure showing antiglare property, formed in order to diffuse a surface reflected light, to stop a regular reflection of outdoor daylight and to prevent a reflection of outside environment ( ) for the purpose of improvement in the visibility of images. As a formation method of optical diffusion layer, a method is mainly used in which a resin layer is formed from a coated layer having a resin containing fine-particles in dispersed state, because fine structure may be easily realized and at the same time excellent productivity may be realized by the method.
However, when a viewing display equipped with a conventional optical diffusion layer is used in bright environment, whitish display screen is obtained to aggravate the color clearness. Especially in use in the open air etc., image often cannot be recognized at all. Moreover, in the case where an LCD is required a high precision, for example having 100 ppi or more, when this LCD is equipped with the above described optical diffusion layer, a portion appears on the LCD surface that has glare, namely a different strength of brightness, probably by an effect of a fine concavo-convex structure formed with particles projected on the surface of the optical diffusion layer, thus leading to decrease in visibility. In order to solve problem of forming of this glare portion, in plenty of cases, layers having a stronger optical diffusion are used, but in this case, clearness leads to be decreased.
The present invention is providing an optical diffusion sheet having an optical diffusion layer in which reflection from outside environment is prevented even in bright environment, such as office environment and outdoors, excellent antiglare property is attained, and whitish display in an image viewing display surface is suppressed to give an excellent clearness, and further screen glare is suppressed even when this is applied to an LCD that is required high clearness.
And this invention also is providing an optical element in which the optical diffusion sheet concerned is provided. Furthermore, this invention also is providing a viewing display equipped with an optical diffusion sheet or an optical element concerned.
As a result of whole-hearted examination to solve the above described subject, the present inventors found out that the above described object might be attained using an optical diffusion sheet having characteristics shown below to complete this invention.
That is, this invention relates to an optical diffusion sheet comprising a transparent substrate and an optical diffusion layer having fine concavo-convex shape on a surface formed by a resin coated layer at least on one side of the transparent substrate,
wherein 60xc2x0 glossiness of a surface of the fine concavo-convex shape is 70% or less, and a haze value of the optical diffusion sheet is 30% or more, and in a viewing display equipped with an optical element comprising the optical diffusion sheet provided thereto, following conditions;
C1/C0xe2x89xa70.2
C2/C0xe2x89xa70.1
xe2x80x83C3/C0xe2x89xa70.1
xe2x80x83are satisfied, when a black and white contrast Cn on the optical diffusion sheet (Cn=(brightness in white display)/(brightness in black display): n is an integer of 0 to 3) is defined as C0 when surface illumination is 0 lux, as C1 when surface illumination is 500 lux, as C2 when surface illumination is 1000 lux, and as C3 when surface illumination is 2000 lux.
In the above described optical diffusion sheet, 60xc2x0 glossiness of a surface with fine concavo-convex shape shows 70% or less, thereby reflection being prevented to give excellent antiglare property. The above described 60xc2x0 glossiness is preferably controlled 60% or less, from the viewpoint of antiglare property. In addition, the above described 60xc2x0 glossiness is 20% or less.
And in the above described optical diffusion sheet of this invention, a haze value is set 30% or more, thus diffusion of transmitted light being controlled. And thereby glare is effectively controlled even when this sheet is applied to an LCD in which high precision is required. When a haze value shows less than 30%, glare is not fully controlled in high precision system. A haze value is preferably set 40% or more. On the other hand, since transmittance will decrease when a haze value becomes higher, a haze value is preferably controlled 60% or less. Especially, a haze value is preferably controlled 40% to 50%.
Moreover, in the above described black and white contrast Cn of the above described optical diffusion sheet of this invention, black and white contrast values of C1, C2, and C3 in case surface illumination is 500 lux, 1000 lux, and 2000 lux to a black and white contrast value C0 in case surface illumination is 0 lux, respectively, are controlled so as to give values of the above described range. Thereby, even when a high surface illumination is given in bright circumstances, such as in office environment and outdoors, excellent antiglare property may be maintained, and simultaneously formation of whitish display in image viewing display surface may be controlled, as a result, leading to improvement in visibility and color repeatability.
A measuring point of luminance concerning the black and white contrast Cn is not especially limited, and generally it is about 30 to 50 cm in a position from the surface of the optical diffusion sheet.
Moreover, each of C1/C0, C2/C0, and C3/C0 is less than 1, but in order to obtain excellent clearness with prevention of whiteness degree, C1/C0 is controlled to give 0.3 or more and preferably 0.4 or more, C2/C0 is controlled to give 0.2 or more and preferably 0.3 or more, and C3/C0 is controlled to give 0.2 or more and preferably 0.3 or more.
It is preferable that a resin coated layer comprises fine particles, and surface concavo-convex form of the resin coated layer is formed by fine particles in the above described optical diffusion sheet. It is also preferable that the resin coated layer is formed with an ultraviolet curable resin.
By using fine particles, a resin coated layer having surface concavo-convex shape may be simply and reliably realized, and at the same time adjustment of the above described glossiness, haze value, and black and white contrast may also be easy. Moreover, the ultraviolet curable resin may form a resin coated layer, namely optical diffusion layer, by easy processing operation such as curing processing by irradiation of ultraviolet.
Besides, this invention relates to an optical diffusion sheet characterized by being provided with a low refractive index layer having a refractive index lower than the refractive index of the resin coated layer on a concavo-convex surface of the resin coated layer of the above described optical diffusion sheet. By existence of this low refractive index layer, antireflection function may be demonstrated and formation of whitish display of the screen in bright circumstances may be suppressed still more effectively.
And the present invention relates to an optical element characterized by the above described optical diffusion sheet prepared on one side or both sides of an optical element. An optical diffusion sheet of the present invention may be used for various kinds of applications, for example, for optical elements.
Furthermore, the present invention relates to a visual display with the above described optical diffusion sheet or the above described optical element provided thereto. An optical diffusion sheet and an optical element of this invention may be used for various kinds of uses. For example, it may be applied for viewing displays and provided on the top face of viewing displays, or on the top face and inside of viewing displays etc.